1. Field of the Invention
The present invention relates to a laminated electronic part, a method for the production thereof, and a dielectric ceramic composition and, more particularly, to a laminated ceramic capacitor having a capacitance as low as from approximately 0.01 pF to 30 pF, suitable for use in a high frequency region of, for example, from several hundreds MHz to several GHz, a method for the production thereof, and a dielectric ceramic composition suitable for use in forming a dielectric ceramic layer of a such laminated electronic part.
2. Description of the Related Art
A variety of substances is known as a material for a dielectric ceramic layer of a laminated ceramic capacitor having a low capacitance for use in a high frequency region. One of such substances is, for example, a dielectric ceramic composition of a (MgaZn(1xe2x88x92a))xSiOx+2 (hereinafter sometimes referred to as xe2x80x9cMZSxe2x80x9d)xe2x80x94Al2O3xe2x80x94SrTiO3 type.
As silica materials for use as part of the MZS, there has generally been employed a crystalline silica material. The major component of the crystalline silica material is SiO2 and this component is very hard. Further, silica material commercially available is one of a crystalline type (xcex1-quartz) and so large in particle size (an average particle size being 3 xcexcm or larger and the maximum size being 8 xcexcm or larger) and so wide in a particle distribution. Moreover, such silica material is so low in a fracture toughness value that it is likely to become an isotropic in shape (an angular shape) when it is finely divided into fine particles by grinding.
In order to obtain finely divided silica powder, there has hitherto been employed a method which involves classifying a supernatant of a suspension of pulverized silica powders. This method, however, can provide fine silica powders having particle sizes as small as approximately 1.5 xcexcm and yet having angular shapes.
Therefore, the MZS powders prepared from such silica powders having such large particle sizes and angular shapes have particles shapes of such silica powders as raw materials still left therein, resulting in angular shapes having coarse or rough surfaces.
When a green sheet is prepared from such MZS of an angular shape, the surface of the resulting green sheet may become coarse or rough, thereby impairing uniformity in the thickness of a dielectric ceramic layer and worsening a distribution of capacitance of laminated ceramic capacitors.
Further, if the green sheet becomes coarse or rough on its surface, an internal electrode to be formed thereon may be made irregular and uneven on its surface, thereby elevating a surface resistance of the internal electrode and impairing frequency properties of factor Q, particularly in a high frequency region.
Moreover, the such green sheet may become narrow in an optimum range of binder amounts, thereby making the binder amount likely to become excessive or too small. If the binder amount would become too large, on the one hand, mold flashing may be occurred to a great extent upon cutting into chips and such flashes cannot be thoroughly removed even if they are to be processed with a barrel. If the binder amount would be too small, on the other, a sheet-binding strength may be decreased, thereby causing the laminated layers to deviate from each other or air to be mixed therein. This may cause a decrease in reliability.
Moreover, the green sheet prepared from the such MZS has a high shrinkage initiation temperature so that the resulting laminated ceramic capacitors may be delaminated readily upon calcination.
A copending patent application is directed to a dielectric ceramic composition of a MZS-Al2O3xe2x80x94SrTiO3 type as represented by the following general formula:
X(MgaZn(1xe2x88x92a))xSiOx+2xe2x80x94YAl2O3xe2x80x94ZSrTiO3
(where symbol a is defined by: 0.1xe2x89xa6axe2x89xa60.8; and
symbol x is defined by: 0.67xe2x89xa6xxe2x89xa61.5);
in which a mole percent ratio of (MgaZn(1xe2x88x92a))xSiOx+2 to Al2O3 to SrTiO3 is located in a region enclosed by a polygon having apexes at points A, B, C and D in a three-component composition map as defined as follows:
This dielectric ceramic composition has superior properties that it can be sintered at temperature of 1,100xc2x0 C. or lower, a dielectric constant xcex5r is as low as 15 or less and no delamination may be caused to occur even when pure Pd is used as material for an internal electrode.
The dielectric ceramic compositions involved in the copending patent application, however, suffers from the difficulty that a somewhat large amount of leak current may be caused when it is used for a dielectric ceramic layer of laminated ceramic capacitors.
The present invention has an object to provide a laminated electronic part having favorable frequency properties of Q value particularly in a high frequency region and least possible leak current of lower than 1.0 xcexcA at 125xc2x0 C. and xe2x88x92600 V.
The present invention has another object to provide a method for the production of a such laminated electronic part.
In a preferred aspect, the present invention has a further object to provide a highly reliable laminated electronic part having a wide range of optimum values at which to use a binder and a plasticizer, good properties to cause less mold flashing upon cutting into laminated chips, a high sheet-binding strength, and favorable properties to cause no or less delamination and to mix air therein.
In another preferred aspect, the present invention has a still further object to provide a method for the production of such a highly reliable laminated electronic part.
In a further preferred aspect, the present invention has an object to provide a laminated electronic part that can be sintered at temperature of 1,100xc2x0 C. or lower, that causes no or little delamination even if pure Pd is used as material for an internal electrode, that has a dielectric constant xcex5r as low as 15 or less, and that has the greatest possible resistance to insulation.
In order to achieve the objects in one aspects as described hereinabove, the present invention provides a laminated electronic part in which one ceramic layer or more ceramic layers is or are laminated alternately with two internal electrodes or more and an interface between the ceramic layer and the internal electrode has a surface roughness in the range of 0 xcexcm to 0.2 xcexcm.
In order to achieve the objects in the other aspects as described hereinabove, the present invention provides a method for the preparation of a such laminated electronic part which comprises the step of calcinating a mixture containing SiO2; the step of preparing a ceramic raw material containing the calcinated material obtained in the previous step; the step of forming a laminated material in such a manner that a non-sintered ceramic layer composed of the ceramic raw material is laminated alternately with a layer having an internal electrode pattern; the step of calcining the laminated material; and the step of forming an outer electrode on the resulting laminated material; wherein there is employed SiO2 having an average primary particle size ranging from 80 nm to 0.5 xcexcm and a particle shape of a generally spherical form.
Moreover, in order to achieve the objects in the still other aspects as described hereinabove, the present invention provides a dielectric ceramic composition comprising a main component and an additive component, the main component being as represented by general formula:
X(MgaZn(1xe2x88x92a))xSiOx+2xe2x80x94YAl2O3xe2x80x94ZSrTiO3
(where symbol a is defined by: 0.1xe2x89xa6axe2x89xa60.8; and
symbol x is defined by: 0.67xe2x89xa6xxe2x89xa61.5);
and the additive component being a compound composed of one or more elements selected from Nb, Ta and W; wherein a mole percent ratio of (MgaZn(1xe2x88x92a))xSiOx+2 to Al2O3 to SrTiO3 is located in a region enclosed by a polygon having apexes at points A, B, C and D in a three-component composition map as defined as follows:
and wherein the additive component is contained at a rate of from 0.01% to 0.2% by mole, when translated into NbO5/2, TaO5/2 or WO3.
These and other objects, features and advantages of the present invention will become apparent in the course of the description which follows, with reference to the accompanying drawings.